The invention concerns an ignition circuit output stage for vehicle occupant protection systems.
In motor vehicles there are trigger devices for protective safety systems such as belt pretensioning systems and airbag systems. By means of an accelerometer, all trigger-relevant data will be evaluated and stored via an analog/digital converter in a microprocessor complete with storage devices. These information data are exchanged and reviewed with an ignition and network control system, even among each other, in order to trigger--when required--the various individual safety systems at the correct point in time. A protective safety device is usually triggered by means of an ignition squib which is driven by an ignition output stage. The squib contains a wire which heats up when a current flows and in this way causes an ignition event to take place. As a result of cost reduction measures, in future the squibs of the gas generators are to be supplied directly from the vehicle battery according to present knowledge and understanding. If the gas generator in question is a particularly safety-relevant gas generator such as the driver airbag, a backup supply capacitor must be integrated additionally; this capacitor continues to supply energy to the system, even if the vehicle battery or its supply line are damaged in the event of a crash.
In the present generation of airbag control devices, the ignition circuit output stages are implemented as "through connect output stages". A through connect output stage is defined as a standard ignition output stage where in the event of an ignition taking place, two switches are closed simultaneously such that the ignition squib(s) is (are) supplied with the entire energy available. Here, for economic reasons, ignition is effected directly from the onboard battery, in some cases also from a spare capacitor kept in reserve. For each safety-relevant ignition a separate capacitor is required as the discharge will be neither regulated nor limited. In fixture generations of airbag control devices, however, only one spare capacitor will be provided for several generators. These systems, therefore, require other ignition circuit output stages complete with "regulated and limited output current mode". In this case only a limited amount of energy will be passed to each ignition squib in a regulated way.
However, the disadvantage of present ignition circuit output stages is that for each mode of operation (output stage mode or regulated and limited output current mode) another output stage is required.